Several devices are presently marketed for measuring the physical properties and integrity of a member, such as a bone non-invasively, in vivo in a patient. Such non-invasive density measuring devices, or densitometers, are typically used to determine cumulative internal damage caused by micro-crushing and micro-fracturing of bone which occurs naturally in humans or animals over time. Such non-invasive density measuring instruments are also used to diagnose the existence and extent of osteoporosis, or loss of bone mineralization, a common chronic problem in elderly humans. The detection, cure and prevention of osteoporosis are currently areas of intense medical interest, particularly in the United States, as a greater number of patients are developing complications resulting from cumulative trabecular bone loss.
One current methodology employed by bone densitometry instruments is based on dual photon energy beams passed through a patient. The sum of the energies passed through the patient at a series of data points are captured by the instrument and, after proper mathematical analysis, are analyzed to locate the bones within the body and to calculate the density of those bones. This technique is sometimes referred to as dual-photon absorptiometry. The commercial current models utilizing this technology are based on dual energy x-ray beams passed through the patient, and are therefore sometimes referred to as dual-energy x-ray apsortiometry, or DEXA instruments. Such instruments are typically used to analyze either the spine or the femur of the patient in whom it is desired to diagnose whether or not osteoporosis might be present. Using such instruments, the average spine bone mineral density (referred to below as "BMD") for elderly normal (non-osteoporotic) subjects without fracture is about 1.0 grams per square centimeter. This value is often considered a threshold for spine fracture, and patients with a bone mineral density value below that 1.0 threshold value are often defined to be osteoporotic or at high risk of fracture. Since bones in a body vary in their extent of mineral density, the threshold of clinical significance must accordingly vary depending on the bone specified.
Various instruments have also been previously proposed to measure such bone density non-invasively using ultrasonic, rather than x-ray measurement techniques. One such ultrasonic densitometer is described in U.S. Pat. No. 4,930,511. In that patent, it is specifically described that the transit time of an ultrasonic pulse transmitted through a living member is related to the bone mineral density of the member. It is further described that the broadband attenuation of the ultrasonic pulse through the member may also be utilized as a measure of mineral density of the bone in vivo. Alternatively, it is described that an ultrasound instrument may calculate both the transit time of the acoustic pulse and the broadband ultrasonic attenuation of the acoustic pulse and may use both results to determine the density of the bone member. The particular instrument described in that patent was constructed to calculate and display a mathematical comparison between measured transit time and normal transit time, or measured attenuation and normal attenuation, as an indication of the density of the member. It is these mathematical comparisons which are presented to the clinical user on the display device described in that instrument.